Hitting Target Original Gravity and Volume

Among the advantages commercial brewers enjoy is their experience with a specific recipe. After brewing numerous batches of the same beer, it’s a relatively simple matter to achieve the same specific gravity, bittering and color, time after time with only small variations. This results in a consistency of flavor, alcohol, body, aroma and appearance, qualities their customers are likely to appreciate.

Homebrewers often wonder how to successfully brew from recipes with which they may have no previous experience. While it may be somewhat difficult to control all of the variables simultaneously, one of the easier — and most important — values to achieve is the target original specific gravity (OG). If you follow the guidelines outlined in this article, you should be able to hit your target OG nearly every time.

Gravity and volume

The keys to hitting the target OG are to know what specific gravity and volume to achieve prior to the boil. The published OG and volume for a recipe are usually measured in the fermenter after the wort is boiled and chilled. Of course, several things occur after the runoff is collected in the kettle and before the wort reaches the fermenter. A very important value is the boiling loss. Because water is evaporated during boiling, the sugars in the wort are concentrated, increasing the specific gravity and decreasing the volume. This results in a pre-boil target gravity that is lower and a volume that is higher than the post-boil values for the recipe.

To calculate the pre-boil volume and gravity, begin by working backward from the published values. Typical homebrew recipes are often expressed in batch sizes of 5 gallons (19 L). This represents the wort that is actually collected in the fermenter and into which the yeast is pitched. To account for boiling and other losses, you will need a significantly larger volume of wort in the kettle.

Boiling losses depend on a number of variables, including kettle geometry, type of burner and vigor of the boil, air temperature, humidity and wind velocity (if brewing outdoors). However, it is possible to determine your typical losses based on experience.

For example, a recipe may be for 5 gallons (19 L) with an OG of 1.048 and a boiling time of 75 minutes (1.25 hours). If your boiling losses are 1.5 gallons (5.7 L) per hour, you will lose 1.9 gallons (7.1 L) during the boil. This would result in a pre-boil target volume of 6.9 gallons (26.1 L):
6.875 = 5 + (1.5 * 1.25)

Once you determine the target pre-boil volume, you can calculate the target pre-boil gravity. The formula is:

Pre-boil specific gravity points = (Post-boil volume * Post-boil gravity points) / Pre-boil volume

Specific gravity “points” are defined as the digits to the right of the decimal point multiplied by 1000; for example, a specific gravity of 1.048 is 48 points.

In our hypothetical recipe with a post-boil target OG of 1.048, a boiling time of 75 minutes and a pre-boil target volume of 6.9 gallons (26.1 L), the formula yields a pre-boil target gravity of 1.035:
34.78 = (5 * 48) / 6.9

A game of percentages

Achieving the pre-boil target gravity is somewhat easier for extract brewers because the extract potential (the specific gravity of a given amount of extract in a given volume of wort — another way of saying the sugar content) is usually known in advance. That is, if x pounds (or grams) of malt extract are added to y gallons (or liters) of water, the specific gravity should be z. For example, the extract potential of light dried malt extract often is given as 1.045. Therefore 5.5 lbs. (2.5 kg) of DME in 7 gallons (26.6 L) of water will yield a pre-boil target gravity of 1.035:

35.36 = (5.5 * 45) / 7.0

All-grain and partial-mash brewers need to account for the varying amount of sugars in the individual grains in the recipe, as well as the mashing technique, to arrive at what is called the “mash efficiency,” that is, the percentage of the total potential sugars they actually extract from the grain into the wort prior to the boil. This will be less than 100 percent.

Grain extract potential data is available from malt producers and/or suppliers. This is the most accurate method because it varies somewhat among individual malt production lots, but brewing texts and software provide reasonably accurate averages for each type of malt.

Calculate the “total potential extract points” for your recipe by multiplying the weight of each grain by its extract potential points, and summing the contribution of each. Then measure the actual pre-boil volume and gravity when you have collected the wort in the kettle. Multiply measured pre-boil gravity points by total volume to calculate “total actual extract points.” Finally, divide the total actual extract points by total potential extract points and multiply by 100 to calculate the mash efficiency in percent.

For example, a basic recipe for 5 gallons (19 L) of pale ale with a published OG of 1.048 might consist of the following:

8.0 lbs. (4.1 kg) pale malt, extract potential 1.039
0.5 lbs. (0.23 kg) medium crystal malt, extract potential 1.034
0.5 lbs. (0.23 kg) dextrin malt, extract potential 1.033

Total potential extract points: (8 * 39) + (0.5 * 34) + (0.5 * 33) = 345.5
Actual measured pre-boil volume: 6.9 gallons (26 L)
Actual measured pre-boil specific gravity: 1.036
Total actual extract points: 6.9 * 36 = 248.4
Mash efficiency: (248.4 / 345.5) * 100 = 71.9 percent

If there are additional fermentables added directly to the kettle (for example, malt extract, sugars or honey), consider their efficiency as 100 percent and add their contribution to the total actual extract points based on their extract potential.

After brewing several batches with the same system, you should be able to determine an average for your typical efficiency. You can use that figure as a multiplier in the previous formula to calculate the pre-boil target gravity. If you know your typical mash efficiency, for example, is 70 percent, the target pre-boil gravity for the recipe above (1.035) can be calculated as follows:
Pre-boil target gravity points = (Total potential extract points * Mash efficiency) / Pre-boil volume

35.05 = (345.5 * 0.70) / 6.9

Tools of the trade

It’s important to measure the volume and gravity with reasonable accuracy. Some brewers make volume marks on the side of the kettle, while others use a previously calibrated sight glass or “dipstick.” Remember to account for the thermal expansion of water as it is heated at temperatures above 39 °F (4 °C). Water and wort increase in volume approximately 2% from 68 °F (20 °C) to 158 °F (70 °C), and 4% from 68 °F (20 °C) to 212 °F (100 °C). If you use a hydrometer to measure specific gravity, you will need to cool the sample to the hydrometer’s reference temperature or adjust the readings to compensate for the sample temperature (there are charts in brewing texts, or software can make the adjustment for you). A refractometer is a handy tool for taking pre-boil gravity readings because the sample is small and cools very quickly.

Once you have measured the pre-boil volume and gravity, these figures can be compared to the target values. Of course, any number of things can occur during the mashing process that may cause you to miss the mark. Don’t despair if this occurs; it happens to all brewers. The secret is to know what measures to take to correct the situation and adjust the gravity or volume accordingly so that you still achieve the target values.

Within reason, the target gravity is more important than the target volume. A little extra volume is desirable in order to allow for various losses after the boil. There are several possible means of adjusting the values, depending on whether the actual volume and gravity are above or below the targets. Diluting the wort with water will increase the volume and decrease the gravity, both before and after the boil. Adding fermentables will increase the gravity. Lengthening the boiling time will decrease the post-boil volume and increase the post-boil gravity, while shortening the boiling time will have the opposite effect.

The right formula for the right job

Let’s examine each of these situations individually and determine the appropriate formulas and measures to make the adjustments.

If the pre-boil gravity is higher than the target, you can dilute the wort by the addition of water. This is especially useful when this is true and the volume is below the target. The amount of water can be determined by the mixing formula Aa + Bb = Cc, where the upper case value is the volume and the lower case value is the specific gravity.

Volume of addition = (Volume before dilution * (Actual gravity points – Target gravity points)) / Target gravity points

For example, if the target pre-boil volume is 6.9 gallons (26.1 L) and the target pre-boil gravity is 1.035, but the actual measurements are 6.8 gallons (25.7 L) and 1.037, dilute with 0.39 gallons (50 oz. or 1.48 L) of water:
0.39 = (6.8 * (37 – 35)) / 35

This will result in a new total pre-boil volume of 7.3 gallons (28 L), and a post-boil volume of 5.4 gallons (20.5 L) if the boiling time is 75 minutes and boiling losses are 1.5 gallons (5.7 L) per hour:
7.3 = 6.9 + 0.39
5.4 = 7.3 – (1.5 * 1.25)

If the pre-boil gravity is lower than the target but the volume is close to it, sometimes the best course of action is to add fermentables to increase the gravity. This can be in the form of malt extract, honey or other sugars. Adding honey or sugar rather than malt extract will decrease the body slightly because these are somewhat more fermentable.

The formula for the addition of fermentables is a combination of the prior mixing formula with the formula for calculating extract points based on the weight and extract potential:

Weight of addition = (Volume of wort * (Target gravity points – Actual gravity points)) / Extract potential points of addition

For example, let’s say the target pre-boil volume is 6.9 gallons (26.1 L) and the target pre-boil gravity is 1.035. The actual pre-boil volume is also 6.9 gallons (26.1 L), but the actual pre-boil gravity is 1.032. Adding 0.46 pounds (7.4 oz. by weight or 209 grams) of light dried malt extract (extract potential 1.045) will increase the gravity to hit the target:0.46 = (6.9 * (35 – 32)) / 45

Dried malt extract has little if any effect on the volume, while liquid malt extract increases the volume slightly because it is typically about 20 percent water.

Adjusting the boiling time affects both the post-boil gravity and the post-boil volume. This also has some effect on beer color and flavor, but these are minor if the time difference is small. Increasing the boiling time might be appropriate for higher gravity beers, while decreasing it might be desirable for lighter colored styles. Increasing the boiling time is less useful if the actual pre-boil gravity is more
than a few points below the target. In that case you would do better by adding fermentables.

The formula for the effect of changing the boiling time on the gravity is:

Increase (or decrease) in boiling time in minutes = (Pre-boil volume * (Target pre-boil gravity points – Actual pre-boil gravity points) * 60) / (Target pre-boil gravity points * Boiling losses per hour)

For example, our hypothetical recipe has a pre-boil target volume of 6.9 gallons (26.1 L) and a pre-boil target gravity of 1.035. If the actual pre-boil volume is 7.0 gallons (26.5 L), the actual pre-boil gravity is 1.033 and boiling losses are 1.5 gallons (5.7 L) per hour, increasing the boiling time by 16 minutes will achieve the post-boil target gravity (OG) of 1.048).

16 = (7.0 * (35 – 33) * 60) / (35 * 1.5)

Thus, if the boiling time specified in the recipe is 75 minutes, the new boiling time is 91 minutes (75 + 16).

In the opposite instance for the same recipe, if the actual pre-boil volume is 6.7 gallons (25.4 L), the actual pre-boil gravity is 1.036 and the boiling losses are 1.5 gallons (5.7 L) per hour, reducing the boiling time by 8 minutes will achieve the post-boil target OG: -7.66 = (6.7 * (35 – 36) * 60) / (35 * 1.5)

In this case, the new boiling time is 67 minutes (75 – 8).

For the effect of the change in boiling time on post-boil volume, the formula is:

Decrease (or increase) in volume = Increase (or decrease) in boiling time in minutes * (Boiling losses per hour / 60)

In the first example above, the corresponding decrease in the post-boil volume is 0.4 gallons (1.5 L), and the new total post-boil volume is 4.7 gallons (17.9 L):
0.4 = 16 * (1.5 / 60)
4.72 = 7.0 – (1.5 * 1.52)

In the example above with the boiling time reduced by 8 minutes, the increase in the post-boil volume is 0.19 gallons (2725 mL), and new total post-boil volume is 5.0 gallons (19 L).
0.19 = 7.66 * (1.5 / 60)
5.02 = 6.7 – (1.5 * 1.12)

It is also possible to adjust the volume and gravity in the fermenter after boiling. Despite the potential effect on body, it is often preferable to add sugar to increase the gravity post-boil rather than malt extract, which generally should be boiled for at least 10 minutes to coagulate proteins that can lead to haze in the beer.

Using these guidelines and with a little practice (and assuming no major brewing mishaps), there is no reason you shouldn’t consistently come within 1 specific gravity point (0.001) of your target OG for every batch you brew. This will go a long way toward improving the consistency of your beers and your ability to successfully brew a wide variety of recipes.

One final note: the equations based on specific gravity are technically only approximations. To rigorously calculate these values, you need to use iterative calculations involving specific gravity and degrees Plato. However, these approximations work well on a homebrew scale and are easy to use.

Issue: July-August 2005